A two-level DC-DC converter series-connected to a DC-AC inverter is often for use in a system with plenty batteries or renewable power supply apparatuses parallel-connected to utility power. Since the virtual work thus produced is seldom absorbed completely by a DC-link in a two-level circuit, second-order ripple occurs to the voltage and input current on the input side of the DC-DC converter, thereby burdening the power components on the input side.
To solve the above problems, the prior art discloses increasing the capacitance level of a DC-link to render the DC-link more capable of absorbing the second-order virtual work and therefore reduce the second-order virtual work level which ends up on a DC input side or discloses increasing the capacitance level on the input side of a DC-DC converter to reduce input voltage ripple. Another conventional technique involves increasing a current loop in a DC-DC converter and reducing input voltage ripple with a current command generated from a voltage loop of an external loop or involves increasing ripple-absorbing circuits in use to thereby transfer the virtual work of the DC-link to the ripple-absorbing circuits, and in consequence the input side of the DC-DC converter does not produce any virtual work and therefore does not have any second-order ripple. However, the aforesaid techniques require increasing circuits or using components with high capacitance levels, thereby incurring high costs.
Accordingly, it is imperative to provide a control method and control device so as to overcome the drawbacks of the prior art.